1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor device and a method of producing the same.
The present application is based on Japanese Patent Applications No. Hei. 11-130475 and 11-266499, which are incorporated herein by reference.
2. Description of the Related Art
A group III nitride compound semiconductor device such as a blue light-emitting device, or the like, was produced by the steps of: growing a buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) on a sapphire substrate by a metal organic chemical vapor deposition method (abbreviated as xe2x80x9cMOCVD methodxe2x80x9d in this specification); and further growing a group III nitride compound semiconductor layer on the buffer layer by the same MOCVD method.
In the MOCVD method, an ammonia gas and gases of group III metal alkyl compounds such as trimethylaluminum (TMA), trimethylgallium (TMG) and trimethylindium (TMI) are supplied onto a substrate heated to an appropriate temperature and are thermally decomposed so that a film of a desired crystal is formed on the substrate. On this occasion, metal organic compounds such as TMA, and so on, which serve as raw-material gases for forming the buffer layer, are expensive. This was a factor of increasing the cost of the group III nitride compound semiconductor device.
If the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) is formed by a method other than the MOCVD method, the use of metal organic compounds such as TMA, TMG, and so on, can be avoided. For example, Japanese Patent Publication No. Hei. 5-86646 has proposed a method in which a buffer layer is formed by a high-frequency sputtering method; group III metal organic compounds are supplied to the buffer layer after the buffer layer is heated (to a temperature of from 800xc2x0 C. to 1000xc2x0 C.) in an atmosphere containing an ammonia gas (ammonia and nitrogen in an embodiment); and then the group III metal organic compounds are decomposed on a heated substrate so that a nitride film thereof is vapor-grown and AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) of the same composition is grown on the buffer layer. Raw materials used for forming the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) by the high-frequency sputtering method are high-purity metallic aluminum and metallic gallium. A mixture gas of argon and nitrogen with these metals as targets is used as a sputter gas. In this case, all the raw materials are inexpensive. Hence, the cost of the device can be reduced compared with the case where expensive metal organic compounds are used as raw materials for forming the buffer layer by the MOCVD method.
Although the inventors of the present invention tried the method disclosed in Japanese Patent Publication No. Hei. 5-86646, the crystallinity of the group III nitride compound semiconductor layer which is formed by the MOCVD method so as to be grown on the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) formed by the high-frequency sputtering method did not satisfy the inventors"" requirements. That is, the crystallinity of the group III nitride compound semiconductor layer obtained in the aforementioned manner was inferior to the crystallinity of the group III nitride compound semiconductor layer which is formed by the MOCVD method so as to be grown on the buffer layer of AlXGa1xe2x88x92XN (0xe2x89xa6Xxe2x89xa61) formed by the MOCVD method.
The inventors of the present application have made one examination after another earnestly to improve the crystallinity of the group III nitride compound semiconductor layer. As a result, the inventors have conceived the prevent invention of the present application.
That is, there is provided a group III nitride compound semiconductor device comprising: a substrate; a first group III nitride compound layer which has a thickness of from 50 xc3x85 to 3000 xc3x85 and which is formed on the substrate by a method not using metal organic compounds as raw materials; and a second group III nitride compound semiconductor layer which is formed on the first group III nitride compound layer.
According to the group III nitride compound semiconductor device configured as described above, the crystallinity of the second group III nitride compound semiconductor layer formed on the first group III nitride compound layer and having a function as the device is improved by an attempt to optimize the thickness of the first group III nitride compound layer which will be a buffer layer.
Incidentally, the thickness of the first group III nitride compound layer is preferably set to be in a range of from 50 xc3x85 to 3000 xc3x85. If the layer is thinner than 50 xc3x85, the layer cannot fulfill the function as a buffer layer. According to the inventors"" examination, there is a risk of the cracking in the first group III nitride compound layer or the second group III nitride compound semiconductor layer formed on the first group III nitride compound layer if each layer is thicker than 3000 xc3x85. That is, either the layer thinner than 50 xc3x85 or the layer thicker than 3000 xc3x85 is undesirable.
On the other hand, Japanese Patent Publication No. Hei. 5-86646 has disclosed the use of a buffer layer having a thickness of from 1000 xc3x85 to 7000 xc3x85. The fact that the upper limit of the film thickness range is set to be 3000 xc3x85 is, however, a novel knowledge which is just acquired now by the inventors and by which the buffer layer or the second group III nitride compound semiconductor layer is prevented from cracking.
The inventors have further made one examination after another earnestly. As a result, they have found that the crystallinity of the second group III nitride compound semiconductor layer formed on the buffer layer is improved when the buffer layer is heated in an atmosphere of a mixture gas containing a hydrogen gas and an ammonia gas in the case where the buffer layer is formed of a group III nitride compound by a DC magnetron sputtering method. Hence, the inventors have conceived the invention.
The configuration of this invention is as follows.
There is provided a method of producing a group III nitride compound semiconductor device, comprising the steps of:
forming a first group III nitride compound semiconductor layer on a substrate by a method not using metal organic compounds as raw materials (step 1);
heating the first group III nitride compound layer in an atmosphere of a mixture gas containing a hydrogen gas and an ammonia gas (step 2); and
forming a second group III nitride compound semiconductor layer on the first group III nitride compound semiconductor layer (step 3).
The configuration of this invention is as follows.
The inventors have photographed an RHEED pattern of an AlN layer as an example of the first group III nitride compound layer formed by the steps up to the step 2 in the aforementioned manner and an RHEED pattern of an AlN layer as an example of the first group III nitride compound layer formed without the step 2 in the aforementioned producing method. As a result of comparison between the two patterns, it has been found that the spot intensity of the former is greater than the spot intensity of the latter.
Accordingly, improvement in crystallinity of the first group III nitride compound layer can be confirmed by execution of the step 2 in which the first group III nitride compound layer is heated in an atmosphere of a mixture gas containing a hydrogen gas and an ammonia gas.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.